1. Field of the Invention
The present invention relates to a spectral measurement apparatus and measurement method utilizing Brillouin scattering which is generated in an optical fiber to judge the state of the temperature of the optical fiber or the strain of the optical fiber (to sense an anomaly).
2. Related Background Art
The Brillouin gain spectrum (referred to as the ‘BGS’ hereinbelow), representing the gain which is obtained as a result of the Brillouin scattering generated in the optical fiber, changes in accordance with the temperature and strain of the optical fiber. Technology that utilizes this change in the BGS to measure the temperature or strain of the optical fiber is known.
The document (Kazuo Hotate and Sean Ong Soon Leng, “A Correlation-Based Continuous-Wave technique for measurement of dynamic strain along an optical fiber using Brillouin scattering with fully distributed ability”, Technical Digest of OFS-15, PD5 (2002)) describes technology for measuring the BGS at respective positions within the optical fiber. In the technology that appears in this document, pumping light and probe light are first caused to enter in opposite directions from the each end of the optical fiber. Thereupon, Brillouin scattering is generated as a result of the pumping light being propagated within the optical fiber and the probe light is subjected to Brillouin gain as a result of the Brillouin scattering. The pumping light and probe light are frequency-modulated sinusoidally and the probe light obtains the Brillouin gain mainly at a correlation peak position where the correlation between the pumping light and probe light increases.
The BGS is measured by fixing the correlation peak position of the optical fiber and sweeping the frequency difference between the pumping light and probe light at the correlation peak position to measure the Brillouin gain received by the probe light. The shape of the BGS changes depending on the temperature and strain of the optical fiber in the correlation peak position and, therefore, by measuring the BGS by establishing the respective positions in the longitudinal direction of the optical fiber in the correlation peak position, the temperature distribution and strain distribution states in the longitudinal direction of the optical fiber can be found.